The present invention concerns the field of suspensions. In particular, it relates to the suspension of vehicles, for example passenger cars. More precisely, it relates to the means used to permit the deflection between two points (a suspended point, for example linked to the body or the chassis of a car, that is to say to the suspended mass, and a non-suspended point, for example fixed to a suspension arm or a wheel carrier, that is to say fixed to the non-suspended mass) while striving to reduce the variations of the force developed by the suspension which are experienced by the suspended point.
It is known that the suspension of a vehicle comprises constructional arrangements by means of which the displacement of a wheel or of wheels of an axle with respect to the body (or with respect to the chassis if the vehicle is provided with a chassis, or between body and chassis if appropriate) is guided, and it comprises means controlling the deflection of each wheel (a wheel is a member belonging to non-suspended members) with respect to the body (any point situated on the body is referred to as a xe2x80x9csuspendedxe2x80x9d point), in order to bear the load while maintaining a certain ground clearance of the vehicle (this is the function of a spring) and in order to prevent the maintenance of the oscillations arising between suspended and non-suspended points following a disturbance (this is the function of a damper).
A spring is calculated to bear the load of the vehicle while maintaining a sufficient ground clearance from the vehicle. Due to the damping function, the wheels equipped with tires tend always to remain in contact with the road when the suspension is stressed by the irregularities of the road. A damper prevents the maintenance of the oscillations arising following a disturbance, in particular the oscillations of the wheel itself, which the tire is unable to attenuate sufficiently rapidly by itself
It is known that the design of a suspension is characterized by the search for a compromise between objectives which have a reputation for being, at least to a large extent, contradictory: comfort and handling. Numerous effort have been made to find increasingly favorable compromises. In the state of the art, suspensions are known which are referred to as active or semi-active suspensions and whose control has now been made possible by progress in electronics. In such suspensions, commands are transmitted by electric or electrohydraulic means in order to modify the flexibility and/or damping characteristics of the suspension in real time. This leads to the addition of an electronic regulation stage, with its sensors and means for calculation, to conventional mechanical, pneumatic or hydraulic devices, as well as solenoid valves or servovalves, in order to transmit the commands for changing the characteristics to the mechanical members responsible for the functioning of the suspension. For example, a valve in a damper has to be opened or closed in order to modify the dissipation of energy which it brings about.
Also known are numerous proposals for active suspensions, such as those described in the patents or patent applications U.S. Pat. No. 5,060,959, U.S. Pat. No. 5,028,073, U.S. Pat. No. 5,066,041, U.S. Pat No. 5,740,039 or EP 0 878 333, to mention only some of them. The diversity of the constructional arrangements and of the control laws proposed compared with the very small number of active suspensions on the market gives an idea of the difficulty of controlling the suspensions effectively. The state of the art comes up against the choice of control parameters, and the correct utilization of a multitude of signals which are thought to be needed for correctly capturing the attitude of a vehicle.
In a particular embodiment proposed in the patent application EP 0 878 333, a regulating parameter called xcex1 is controlled in real time in order to adjust the suspension between a pure contour-following or spring-accompanying behaviour (xcex1=1) and a pure damping behaviour (xcex1=0). This mode of operation is useful in particular for damping the vibrations of a tire. It is proposed to detect the occurrence of a resonance of the tire in order to rapidly give xcex1 a value sufficiently below 1 (for example according to a law of linear variation as a function of the amplitude) so as to damp the vibrations of the non-suspended mass.
The aim of the invention is to propose another approach, which makes the detection of a resonance of the non-suspended mass much less useful, for obtaining a high level of operational safety of the vehicles and great comfort.
The invention proposes a suspension device, for controlling the vertical deflection between a suspended mass and a non-suspended mass, comprising an elastic link mounted between the suspended mass and the non-suspended mass, the said elastic link including a spring and comprising a reversible electric actuator acting in parallel with the spring for controlling the relative-deflection movements between suspended mass and non-suspended mass, Zcr being the distance between the suspended mass and the non-suspended mass, the said device comprising means for controlling the electric actuator in order, from an observed variation of the distance Zcr between the suspended mass and the non-suspended mass, due to a deflection of the elastic link, that the electric actuator develops a force F having a mass compensation component F1=Caxc2x7Mrxc2x7Acr oriented like the acceleration Acr of the deflection of the non-suspended mass with respect to the suspended mass, with Mr being the non-suspended mass and Ca a coefficient, and having a damping component F2=xe2x88x92Cvxc2x7Vcr opposing the speed Vcr of the said deflection, Cv being a coefficient.
By convention, xe2x80x9crevolving assemblyxe2x80x9d is the term used for all the members which, for a given suspension configuration, are customarily arranged in the non-suspended mass. For a suspension of the type described in the application EP 0 878 332, the non-suspended mass comprisesxe2x80x94besides all the members rotating about the axis of rotation of the wheelxe2x80x94the wheel carrier, the bar, the brake calliper and the electric drive motor (the suspension motor is not part of it). The invention proposes to develop a force component in the electric actuator which takes account of the non-suspended mass, thereby tending to prevent the accumulation of energy therein.
In addition and with preference, the electric actuator is controlled so as to develop a spring-accompanying component, creating a force oriented like the relative displacement between body and wheel, so as to cancel out the force variations due to the deflection variations. The electric actuator of the device according to the invention is, in this preferable case, controlled by the said control means so that the electric actuator develops a force F additionally having a spring-accompanying component F3=Cpxc2x7Kxc2x7Zcr, oriented opposite to the variation of the force developed by the spring and due to the variation of the distance Zcr, K being the stiffness of the spring and Cp a coefficient.
The role of the electric actuator is to limit as far as possible the force variations experienced by the body. If there were no force variation, there would be no acceleration, and thus no movement of the body vertically, so that the comfort would be very great. The electric actuator continuously acts as a spring corrector so as to radically reduce the force variations which the spring would otherwise impose on the suspended mass under the action of external disturbances, while preventing an unacceptable accumulation of energy in the non-suspended mass. The function of this spring corrector consists in modifying the reaction force which the spring would have naturally developed in response to a disturbance by applying a correction strategy aiming to obtain the best-possible comfort while preserving the safety.
The invention will be better understood from the following description of an example given on a non-limiting basis, with reference to the appended drawings.